JP3638557B2 - Cryogenic cooling method and apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a cryogenic cooling method and apparatus, and in particular, a small and inexpensive refrigeration suitable for use in a refrigerator in which a Joule-Thomson (JT) circuit is added to the refrigerator to lower the refrigeration temperature. The present invention relates to a cryogenic cooling method and apparatus capable of realizing a machine.
[0002]
[Prior art]
A commonly used refrigerator, such as a Gifford McMahon (GM) refrigerator, a pulse tube refrigerator, a Stirling refrigerator, etc., is composed of two stages. When the GM refrigerator 20 is operated as shown in FIG. The temperature of the first stage (referred to as the first stage) 21 is, for example, 77K (determined by the model used and the size of the load, and should be 45 to 90K), and the second stage (referred to as the second stage) 22. The temperature can be cooled to, for example, 10K or less. In particular, the temperature of the second stage 22 can be cooled to about 2.5 K by changing the cold storage material put into the displacer inside the GM refrigerator according to the application.
[0003]
In the figure, 10 is an object to be cooled, 12 is a vacuum vessel for heat insulation, and 18 is a compressor of a GM refrigerator.
[0004]
However, the GM refrigerator alone cannot normally be cooled below 2.5K. Also, the refrigeration capacity at this time becomes zero. Usually, in the case of the GM refrigerator 20 often used, 3 to 50 W (when the temperature is 77 K) at the first stage 21 and 0.1 to 1.5 W (when the temperature is 4.2 K) at the second stage 22 ) Select a model with an appropriate capacity depending on the application.
[0005]
However, below 4K, especially below 3K, the refrigerating capacity is drastically reduced, and the object to be cooled cannot be cooled practically. For this reason, when cooling to a temperature lower than 3K is necessary, the GM refrigerator 20 alone cannot cope with it, so the GM refrigerator 20 is used as a primary cooling device as shown in FIG. A refrigerator is configured by adding a JT circuit 30 (see JP-A-10-73333 and JP-A-11-108476).
[0006]
The JT circuit 30 includes three coolers 31, 32, 33, three heat exchangers 34, 35, 36, and a JT valve 38. These are provided in the vacuum vessel 12 for heat insulation. It is put. Here, the name JT is so called because it adiabatically expands the helium gas and uses cooling by the Joule-Thomson effect. FIG. 2 shows an example of a temperature (T) -entropy (S) diagram at a low pressure of 0.01 MPa and a high pressure of 1 MPa in a GM refrigerator to which a JT circuit is added.
[0007]
In the case of a cooler to which the JT circuit 30 is added, the high pressure (1-2MP) helium gas from the compressor 42 enters the first heat exchanger 34 and exchanges heat with the returning low-temperature helium gas. , And cooled to about 90K. Next, it is cooled to about 75K by the first stage cooler 31 provided in the first stage 21 of the GM refrigerator 20. This first stage cooler 31 compensates for the loss mainly caused by the first heat exchanger 34 (higher efficiency is better, but it cannot be made 100%). Next, the heat is exchanged with the low-temperature helium gas that enters the second heat exchanger 35 and returns, and is cooled to about 15K. The two-stage cooler 32 provided in the second stage 22 of the GM refrigerator 20 is cooled to about 10K. Next, the heat enters the third heat exchanger 36 and exchanges heat with the returning low-temperature helium gas, and is cooled to about 6K.
[0008]
Helium gas has a reversal temperature at about 30 K, and there is a region (determined by temperature and pressure) in which the temperature decreases by expanding below this temperature. Therefore, when the high-pressure helium gas cooled to about 6K is expanded to atmospheric pressure (0.1 MPa) or less by adiabatic free expansion, a part of the gas becomes liquid. The amount of liquefaction depends on the temperature and pressure before expansion and the pressure after expansion.
[0009]
When helium is liquefied after expansion, the temperature is determined by the vapor pressure of the liquid helium (same as the pressure after expansion) (referred to as equilibrium temperature).
[0010]
Helium gas is expanded through a JT valve 38 which is an expansion valve. Since this expansion is adiabatic free expansion, the expansion valve is simply a throttle valve (usually using a small needle valve). The expanded helium cools the object 10 to be cooled by the load cooler 33 (receives heat), the liquid evaporates, and the third heat exchanger 36, the second heat exchanger 35, and the first heat exchanger 34 It returns in order, returns to room temperature, and is compressed by the vacuum pump 40 and the compressor 42 again.
[0011]
In the figure, reference numeral 44 denotes an overcompression preventing protective device for the vacuum pump 40.
[0012]
[Problems to be solved by the invention]
Thus, when obtaining a temperature lower than 4.2 K, it is necessary to reduce the low pressure of the JT circuit 30 to about 0.1 MPa (1 atm) or less in absolute pressure. Since a normal compressor cannot compress from a very low suction pressure, the gas circulating through the JT circuit 30 is first compressed to atmospheric pressure using a vacuum pump (or a compressor capable of low-pressure suction) 40. The pressure is then increased to a required pressure (1 to 2 MPa) with a normal compressor 42. Here, as the vacuum pump 40, a type called an oil rotary pump is generally used. When the suction pressure is lower, a mechanical booster such as a root type supercharger is combined.
[0013]
The pressure to be depressurized is determined by the required temperature, 24.4 KPa at 3K, 3.17 KPa at 2K, 1.67 KPa at 1.8 K, and decreases rapidly as the temperature decreases. Also, as already described, the refrigerator has heat exchangers 34, 35, and 36, and the return low-pressure gas must pass through them, causing a pressure loss. For this reason, the suction pressure of the vacuum pump 40 is further reduced, and when the cooling temperature is lowered, the required pumping speed of the vacuum pump is rapidly increased, and the size is also increased.
[0014]
Thus, in the conventional method, a combination of the vacuum pump 40 and the compressor 42 is necessary for the circulation of the gas in the JT circuit 30. When a protective device 44 that prevents the discharge pressure of the vacuum pump 40 from rising excessively or a mechanical booster is used, the protective device is also required to prevent the motor from burning. It was large, complex and expensive.
[0015]
The present invention has been made to solve the above-mentioned conventional problems, and as in the conventional case, the maximum capacity of the refrigeration machine to be used is not used, and the refrigeration capacity at a low temperature is not used to the maximum. Therefore, it is an object of the present invention to realize a small and inexpensive refrigerator that prevents a vacuum pump and a compressor at room temperature from becoming large when there is a relatively large margin.
[0016]
[Means for Solving the Problems]
The cooling target is various sensors and pre-cooling circuits, and the required refrigerating capacity is small, for example, less than 1W, and when a refrigerator that can be cooled to 4K, such as a GM refrigerator, can be used, that is, the refrigerator to be used can be cooled to 4K. Thus, when there is a relatively large margin for the load, the high pressure of the JT circuit can be reduced to about 0.1 MPa (atmospheric pressure). The present invention has been made paying attention to this point, and by liquefying the refrigerant in the JT circuit, the vacuum pump and the compressor at room temperature are simplified, and a small and inexpensive refrigerator is realized. is there.
[0017]
That is, the present invention relates to the high pressure of the Joule-Thompson circuit for adiabatic free expansion of the refrigerant primarily adiabatic free expansion by adiabatic free expansion of the refrigerant first cooled in the refrigerator and further cooling by the Joule-Thomson effect to obtain a cryogenic temperature The pressure is set to a predetermined pressure slightly higher than the atmospheric pressure, and the temperature of the two-stage cooler disposed in the second stage of the refrigerator is lower than the refrigerant condensation temperature determined by the high pressure of the Joule-Thomson circuit. Thus, the refrigerant is condensed by the two-stage cooler to solve the above problem.
[0018]
The present invention also relates to a refrigerator for primary cooling of the refrigerant, and Joule Thomson for adiabatic free expansion of the refrigerant primarily cooled by the refrigerator and further cooling by the Joule Thomson effect to obtain a cryogenic temperature. A cryogenic cooling apparatus comprising a circuit, a vacuum pump disposed in the Joule-Thomson circuit and capable of discharging to a pressure slightly higher than atmospheric pressure, and a refrigerant disposed in a two-stage stage of the refrigerator The above-mentioned problem is similarly solved by providing a two-stage cooler that cools and condenses to a condensation temperature determined by the high pressure of the Joule-Thomson circuit.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention that are suitable for use in cooling and precooling circuits for various sensors will be described in detail below with reference to the drawings.
[0020]
In the first embodiment of the present invention, in a refrigerator similar to the conventional one, as shown in FIG. 3, the high pressure of the JT circuit 30 is set to a predetermined pressure (for example, 0.2 MPa or less) slightly higher than the atmospheric pressure. In the third heat exchange, the helium is cooled to a condensation temperature determined by the high pressure of the JT circuit 30 by the two-stage cooler 32 disposed in the two-stage stage 22 to be condensed and condensed. The vessel (36 in FIG. 1) is omitted. The other points are the same as in the conventional example shown in FIG.
[0021]
As the vacuum pump 40, for example, an oil rotary vacuum pump whose discharge pressure is allowed to a pressure slightly higher than atmospheric pressure (up to 0.2 MPa) can be used.
[0022]
Hereinafter, the operation of the present embodiment will be described.
[0023]
The high pressure helium gas (pressure slightly higher than the atmospheric pressure of 0.1 MPa) from the vacuum pump 40 enters the first heat exchanger 34 and exchanges heat with the returning low temperature helium gas, and is cooled to about 60K. The Next, it cools to about 40K with the 1st stage cooler 31 arrange | positioned at the 1st stage 21 of the GM refrigerator 20. The first stage cooler 31 mainly compensates for the loss generated in the first heat exchanger 34.
[0024]
Next, it enters the second heat exchanger 35 and exchanges heat with the returning low-temperature helium gas, and is cooled to about 8K. Thereafter, it is cooled to 4K by a two-stage cooler 32 provided in the second stage 22. At this time, if the high pressure is higher than 0.1 MPa (1 atm), the helium gas is condensed here to become a liquid. In addition, in the conventional refrigerator, since the temperature of this part is as high as about 10K, it does not liquefy.
[0025]
Thus, in the present invention, since the liquid is already in front of the JT valve 38, the conventional third heat exchanger (36 in FIG. 1) is not necessary.
[0026]
The helium condensed into the liquid in the two-stage cooler 32 passes through the JT valve 38 and expands to a low pressure. At this time, part of the liquid evaporates, and the temperature drops to a temperature determined by the pressure after expansion.
[0027]
The expanded helium cools the object to be cooled 10 through the load cooler 33 (receives heat), the liquid evaporates, and returns to the second heat exchanger 35 and the first heat exchanger 34 in this order. Return to the vacuum pump 40 at room temperature.
[0028]
Here, the temperature of the second stage 22 of the GM refrigerator 20 is operated so as to be about 4K, and a gas corresponding to the temperature is supplied to the JT circuit 30. At this time, when the high pressure of the JT circuit 30 is slightly higher (about 0.01 MPa) than the equilibrium pressure determined by the temperature of the second stage 22 of the GM refrigerator 20, the two-stage cooler 32 provided in the second stage 22 is used. Then, helium is condensed into a liquid. For this reason, during such operation, if the high pressure of the JT circuit 30 is set to a pressure slightly higher than 0.1 MPa (atmospheric pressure), it is not necessary to increase the pressure further. Accordingly, although the refrigerating capacity is slightly reduced, it is not necessary to suck the low-pressure helium gas with the vacuum pump 40 and further compress the discharged gas with the compressor 42 as shown in FIG.
[0029]
Of the functions of the JT valve 38 as the expansion valve and the flow rate adjustment valve, only the function as the expansion valve is required in the low temperature part, so as in the second embodiment shown in FIG. At the position of the JT valve (38 in FIG. 3), an expansion valve (also referred to as a JT valve) 50 having the function of the primary pressure adjusting valve shown in FIG. 5 is provided, and the flow rate adjustment as shown in FIG. A valve 52 or a fixed orifice may be provided to stabilize the flow rate so that the refrigerator can be operated very stably.
[0030]
As the expansion valve 50, for example, a primary pressure adjusting valve that works to keep the pressure on the inlet side (referred to as primary pressure) at a certain set pressure as shown in FIG. 5 can be used. When the inlet pressure increases, the primary pressure adjusting valve 50 extends the bellows 50B against the spring 50C to raise the tip of the needle 50N, and opens the valve to lower the inlet pressure. Conversely, when the inlet pressure is lowered, the bellows 50b is contracted by the action of the spring 50C to lower the tip of the needle 50N, and the valve is throttled to prevent the inlet pressure from being lowered.
[0031]
In the figure, 50A is a pressure adjusting screw for adjusting the pressure via a spring 50C, and 50S is a stem lengthened to reduce intrusion heat.
[0032]
Thus, the primary pressure regulating valve 50 keeps the inlet pressure constant regardless of the flowing flow rate. The detection of the inlet pressure and the function as an actuator are performed by the bellows 50B or the diaphragm provided in the upper part. The adjustment pressure is set by adjusting the force of the spring 50C for suppressing the bellows 50B or the diaphragm with the pressure adjustment screw 50A.
[0033]
Hereinafter, the operation of the primary pressure adjusting valve 50 will be described in detail.
[0034]
The gas inlet communicates with the inside of the bellows 50B at the upper portion by the valve stem 50S. A long needle 50N is attached to the upper lid 50U of the bellows 50B. A spring 50C is provided on the bellows 50B to push the bellows 50B.
[0035]
Now, when gas of a certain pressure is supplied to the inlet, the inside of the bellows 50B becomes the supply pressure. When the effective area of the bellows 50B is A and the differential pressure between the inside and outside of the bellows 50B is P, the force F generated in the bellows 50B is
F = A × P
It becomes. When the supply pressure is high and this force F is greater than the force of the spring 50C pushing the bellows 50B, the bellows 50B extends and the needle 50N is lifted. This opens the needle valve and allows the supply gas to escape from the outlet. Conversely, when the supply pressure is low, the needle valve is closed. In this way, the valve keeps the inlet pressure constant.
[0036]
As described above, since the JT valve 50 functions to keep the high-pressure side pressure of the JT circuit 30 constant, the gas source (vacuum pump) 40 having a slightly higher pressure (0.05 to 0.1 MPa) is used as a refrigerator. Since the JT valve 50 operates to keep the pressure on the high pressure side constant when the gas is flowed to the flow, if there is no flow rate adjustment valve, the flow rate will continue until it is limited by the pressure loss of the heat exchanger and piping. growing.
[0037]
Since this does not work properly as a refrigerator, the valve 52 that adjusts the flow rate is inserted between the gas source and the refrigerator inlet, and the optimal gas flow rate for the operation of the refrigerator can be used to stabilize the refrigerator. Can drive to.
[0038]
Since the flow rate adjusting valve 52 is at room temperature, any type can be used as long as the flow rate can be adjusted, and a commercially available valve can be used.
[0039]
An example of the flow rate adjusting valve 52 is shown in FIG. In the figure, 52N is a needle, 52A is a screw for adjusting the flow rate by moving the needle 52N up and down, and 520 is a 0 ring.
[0040]
Since the flow rate adjusting valve 52 is placed at room temperature, it can flow a determined flow rate much more stably than a valve placed at a low temperature. Moreover, even when it is desired to make it more stable, it can be manufactured at a lower cost than a low-temperature automatic adjustment valve. On the other hand, when it is not necessary to strictly adjust, it is possible to fix using an orifice.
[0041]
In this way, by making the JT valve placed at a low temperature the primary pressure regulating valve 50 having the function of an expansion valve, the delicate adjustment required by the conventional JT valve becomes unnecessary. Further, the flow rate can be adjusted by adjusting the flow rate adjustment valve 52 placed at room temperature, and much better stability can be obtained as compared with the adjustment valve at a low temperature. It is also possible to use an orifice in addition to the valve.
[0042]
Furthermore, even if the refrigerator is not cooled, if the flow rate is adjusted first when the refrigerator is operated, the flow will flow at a stable set flow rate thereafter. For further stabilization, it is possible to measure the flow rate with a flow meter and perform feedback control to adjust the flow rate to a constant flow rate with a relatively simple circuit.
[0043]
As a result, according to the second embodiment, the refrigerator loaded with the JT circuit can be operated very stably.
[0044]
In each of the above embodiments, a GM refrigerator is used as the refrigerator. However, the type of the refrigerator is not limited to this, and may be a pulse tube refrigerator or a Stirling refrigerator. Further, the refrigerant is not limited to helium, and the application target is not limited to the cooling or precooling circuit of various sensors, and can be applied to, for example, a helium liquefier or a hydrogen liquefier.
[0045]
【The invention's effect】
According to the present invention, the pressure on the high pressure side of the JT circuit only needs to be slightly higher than the atmospheric pressure, and a compressor for compressing from the atmospheric pressure to a higher pressure is unnecessary, and the gas circulation (decompression and compression) of the JT circuit is a vacuum pump. Can be performed simply to simplify the gas (compression) circulation device at room temperature. Therefore, for example, by using an oil rotary vacuum pump whose discharge pressure is allowed to be slightly higher than atmospheric pressure (up to 0.2 MPa), the refrigerator can be manufactured in a small size and at low cost.
[0046]
Furthermore, since the third heat exchanger can be omitted, the number of components of the refrigerator main body can be reduced, and the refrigerator can be manufactured at low cost. Further, since the pressure loss in the low pressure return flow path of the JT circuit is reduced by the reduction in the heat exchanger, the cooling temperature can be lowered accordingly.
[Brief description of the drawings]
FIG. 1 is a flowchart showing the configuration of a conventional refrigerator with a JT circuit added. FIG. 2 is a TS diagram showing the operation of the conventional refrigerator. FIG. 3 is a first embodiment of a refrigerator according to the present invention. FIG. 4 is a flowchart showing the configuration of the second embodiment of the refrigerator according to the present invention. FIG. 5 is an example of an expansion valve (primary pressure adjusting valve) used in the second embodiment. Sectional view showing [FIG. 6] Cross-sectional view showing an example of the same flow control valve [Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Cooled object 12 ... Vacuum container 18 ... GM refrigerator compressor 20 ... GM refrigerator 21 ... First stage 22 ... Second stage 30 ... JT circuit 31 ... First stage cooler 32 ... Two stage cooler 33 ... Load coolers 34, 35, 36 ... heat exchanger 38 ... JT valve 40 ... vacuum pump 42 ... JT circuit compressor 44 ... vacuum pump overcompression protection device 50 ... primary pressure regulating valve (expansion valve, JT valve)
52 ... Flow rate adjustment valve

Claims (2)

When the refrigerant primarily cooled in the refrigerator is adiabatic free expansion and further cooled by the Joule-Thomson effect to obtain a cryogenic temperature,
The high pressure of the Joule-Thompson circuit for adiabatic free expansion of the refrigerant is set to a predetermined pressure that is slightly higher than atmospheric pressure,
The temperature of the two-stage cooler disposed in the second stage of the refrigerator is lower than the condensation temperature of the refrigerant determined by the high pressure of the Joule-Thomson circuit, and the refrigerant is condensed in the two-stage cooler. A cryogenic cooling method characterized by being made to do so. A refrigerator for primary cooling of the refrigerant;
In a cryogenic cooling device comprising a Joule-Thomson circuit for adiabatic free expansion of the refrigerant primarily cooled by the refrigerator and further cooling by the Joule-Thomson effect to obtain a cryogenic temperature,
A vacuum pump disposed in the Joule-Thomson circuit and capable of discharging to a pressure slightly higher than atmospheric pressure;
A two-stage cooler disposed on the two-stage stage of the refrigerator to cool and condense the refrigerant below a condensation temperature determined by the high pressure of the Joule-Thomson circuit;
A cryogenic cooling device comprising:

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